Add CODEOWNERS

[anuraaga]

Currently, contributors have to search through a longish list of reviewers for the three main maintainers when requesting reviews. With a CODEOWNERS file, the maintainers will be automatically added to PRs as reviewers.

It is also possible to add directory-specific owners, which I went ahead and did for gRPC.

[codecov]

Codecov Report

Merging #1098 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1098      +/-   ##
==========================================
- Coverage   72.39%   72.27%   -0.12%     
==========================================
  Files         507      507              
  Lines       22377    22377              
  Branches     2772     2772              
==========================================
- Hits        16199    16174      -25     
- Misses       4703     4729      +26     
+ Partials     1475     1474       -1

Impacted Files	Coverage Δ
...armeria/server/tomcat/Tomcat85ProtocolHandler.java	50% <0%> (-13.64%)	⬇️
.../linecorp/armeria/server/tomcat/TomcatService.java	59.32% <0%> (-7.63%)	⬇️
...meria/common/stream/RegularFixedStreamMessage.java	82.97% <0%> (-6.39%)	⬇️
...linecorp/armeria/internal/Http2GoAwayListener.java	76% <0%> (-4%)	⬇️
...inecorp/armeria/client/grpc/ArmeriaClientCall.java	76.92% <0%> (-1.71%)	⬇️
...com/linecorp/armeria/internal/grpc/GrpcStatus.java	44.44% <0%> (+11.11%)	⬆️

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[trustin]

Awesome. Thanks!

[minwoox]

Awesome!

[minwoox]

Thanks @anuraaga